1. Technical Field
The present invention regards a system for communicating over a power cable.
2. Background Art
There is in many situations a need for communication over large distances. Often, such communication needs coincides with a need for transferring power.
For example in petroleum wells there is a need for communication from topside to downhole for transferring control signals to downhole equipment and from downhole to topside, for example for transferring monitoring data from downhole equipment. In petroleum wells the distances are large and the power sources are often high voltage power sources.
Other examples of areas having communication needs are drilling operations for tunnels or wells, monitoring galleries in mines, etc.
Traditionally such communication has been based on dedicated cables or traditional modem technology. As there often are long distances through a rough environment and the power cable is radiating a lot of ripple noise, traditional modem technology often get problems. In hot environments optical fiber communication is not an issue due to limited performance of lasers at high temperature.
U.S. Pat. No. 5,995,020 regards a downhole power and communication system for operating a tool string in a well. The system comprises an apparatus for transmitting electricity between a downhole well tool and the well surface. A controller is located at the well surface, a receiver module is engaged with the well tool, and a conductor is connected to the controller and the receiver module. The controller may include a communication transmitter and a modulator operable to generate an electric signal representing at least two states and/or a data bit representing an address. The address is used to connect power to a desired tool.
US 2007/0202839 regards a method of transmitting data over a regular power line of a switch mode power supply system. The data are transmitted by frequency modulating the power signal. This method is not adapted for use in petroleum wells and the communication is from the power source to the power user only.
US2007/0286305 regards a transmitting apparatus of digital signals to a user on a line in which a supply signal of electronic devices is present. One embodiment comprises a switching converter and means for varying the switching frequency of the converter on the bases of a signal to be transmitted.